Robinson–Foulds metric
The Robinson–Foulds or symmetric difference metric, often abbreviated as the RF distance, is a simple way to calculate the distance between phylogenetic trees. It is defined as (A + B) where A is the number of partitions of data implied by the first tree but not the second tree and B is the number of partitions of data implied by the second tree but not the first tree (although some software implementations divide the RF metric by 2 and others scale the RF distance to have a maximum value of 1).